Catalytic Decomposition of Long‐Chain Olefins to Propylene via Isomerization‐Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene‐Ruthenium Olefin Metathesis Catalysts

Abstract One of the most exciting scientific challenges today is the catalytic degradation of non‐biodegradable polymers into value‐added chemical feedstocks. The mild pyrolysis of polyolefins, including high‐density polyethylene (HDPE), results in pyrolysis oils containing long‐chain olefins as major products. In this paper, novel bicyclic (alkyl)(amino)carbene ruthenium (BICAAC−Ru) temperature‐activated latent olefin metathesis catalysts, which can be used for catalytic decomposition of long‐chain olefins to propylene are reported. These thermally stable catalysts show significantly higher selectivity to propylene at a reaction temperature of 75 °C compared to second generation Hoveyda–Grubbs or CAAC−Ru catalysts under ethenolysis conditions. The conversion of long‐chain olefins (e.g., 1‐octadecene or methyl oleate) to propylene via isomerization‐metathesis is performed by using a (RuHCl)(CO)(PPh3)3 isomerization co‐catalyst. The reactions can be carried out at a BICAAC−Ru catalyst loading as low as 1 ppm at elevated reaction temperature (75 °C). The observed turnover number and turnover frequency are as high as 55 000 and 10 000 molpropylene molcatalyst −1 h−1, respectively.


General information
All metathesis reactions were conducted under nitrogen atmosphere using Schlenk-technique or under argon using a glovebox. The reagents and solvents (Aldrich) including deuterated solvents (Eurisotop) were used as received. 1-Octadecene (Aldrich) was filtered through a short pad of activated alumina in the glovebox. Catalyst ultraNitroCat (1) and ultraCat (3)  GC-MS analyses were carried out using a Shimadzu GC-MS-QP2010 instrument fitted with an Rxi-5Sil MS column coupled with a quadrupole mass filter with pre-rods. The gaseous reaction products were analyzed on-line by a Shimadzu GC-2010 gas chromatograph (GC) equipped with a 50-m HP-PLOT-Fused Silica column (Al2O3, KCl), flame ionization detector (FID).
For the high resolution mass spectrometric measurements (HRMS) a Maxis II type Qq-TOF MS instrument (Bruker Daltonics, Bremen, Germany) with an electrospray ion-source were used. The spray voltage was kept at 3.5 kV and N2 was used as the drying (200 °C, 4.0 L/min) and nebulizer gas (0.5 bar). The mass spectra were recorded by a digitizer at a sampling rate of 2 GHz. The mass accuracy of the instrument is better than 600 ppb (internal calibration) and the resolution power is higher than 40000 at m/z 400 (fwhm). The MS spectra were calibrated internally with sodium formate clusters formed in-situ under electrospray and evaluated by means of the Compass DataAnalysis 4.4 software from Bruker Daltonics (Bremen, Germany). The samples were dissolved in dichloromethane (DCM) and then diluted with methanol (MeOH) (MeOH/DCM : 9/1 V/V) to obtain sample concentrations of 0.01-0.04 mg/mL. X-ray-quality crystals of 5, 14, 18 and 20 were grown by slow evaporation or cooling of hexane or DCM solutions. A crystal well-looking in polarized light microscope was fixed under a microscope onto a Mitegen loop using high-density oil. Diffraction intensity data were collected at room temperature (295-300 K) using a Bruker-D8 Venture diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with INCOATEC IμS 3.0 (Incoatec GmbH, Geesthacht, Germany) dual (Cu and Mo) sealed tube micro sources and a Photon II Charge-Integrating Pixel Array detector (Bruker AXS GmbH, Karlsruhe, Germany) using Mo Kα (λ = 0.71073 Å) radiation, in case of 5 Cu Kα. radiation was applied because of the rather long unit cell axis. High multiplicity data collection and integration were performed using APEX3 (version 2017.3-0, Bruker AXS Inc., 2017, Madison, USA) software. Data reduction and multi-scan absorption correction were performed using SAINT (version 8.38A, Bruker AXS Inc., 2017, Madison, USA). The structure was solved using direct methods and refined on F 2 using the SHELXL program 1 incorporated into the APEX3 suite. Refinement was performed anisotropically for all non-hydrogen atoms. Hydrogen atoms were placed into geometric positions The CIF file was manually edited using Publcif software, 2 while graphics were prepared using the Mercury program. 3 The results for the X-ray diffraction structure determinations were very good according to the Checkcif functionality of PLATON software (Utrecht University, Utrecht, The Netherlands), 4 and structural parameters such as bond length and angle data were in the expected range. Deposition Numbers 2128047-2128050 for 14, 18, 5 and 20 (respectively), as well as 2144205 and 2144206 for the other enantiomer of 18 and the inversion twin crystal of 18 from the racemic conglomerate and 2144599 for 15, respectively contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
The synthetic procedure reported by the Bertrand group was slightly modified. At step (i), molecular sieves were used instead of magnesium sulfate as drying agent. Molecular sieves have greater drying intensity than the magnesium sulfate and its acidity replaces the para-toluenesulfonic acid catalyst. At step (ii), lithium diisopropylamide (LDA) solution was used instead of butyllithium solution.
The basicity of the LDA is also sufficient for the deprotonation of the imine, and does not require intensive cooling (0 °C is enough instead of -78 °C). At step (iii), 1,4-dioxane was used as solvent instead of diethyl ether. This change was applied for only safety reasons. The indicated alternative for generating hydrogen chloride in-situ by reacting dry methanol with acetyl chloride afforded the products (10-13) in same yields as in case of commercially available dioxane-HCl solutions.

Synthesis of carbene precursors 10-13
The aldehyde (6, Trivertal, Fluorochem) and anilines 7b-d (Merck) used in reaction step i are commercially available. Compound 7a was synthetized following the procedure described in a paper published by our group recently. 7 As a representative example, herein the synthesis of the novel Me BICAAC Et2 ligand's precursor (12) is reported in detail.

Synthesis of alkylated imine 9c
Synthesis of imine 9c In a Schlenk-flask, imine 8c (8.08 g, 30.00 mmol) was dissolved in dry THF (20 mL). Lithium-diisopropylamide (LDA) solution (2 M in THF, 60.00 mmol, 30 mL) was diluted with another crop of dry THF (20 mL), than it was cooled to 0 °C using an ice-bath. To this stirred LDA solution, the imine's solutin was added dropwise, then the resulted reddish-brown mixture was allowed to warm up to room temperature. Then the mixture was stirred for additional 4 hours. (Note: less than 4 hours of deprotonation gradually decreases the reaction's yield.) The solution was cooled down to 0 °C, and methyl iodide (3.73 mL, 60.00 mmol) was added dropwise resulting a yellow suspension. The mixture was stirred overnight (16 h), then the reaction's progress was checked with GC-MS. Then distilled water and hexane were added to the suspension, until the precipitated inorganic salt dissolved. The organic layer was washed three times with water (40 mL each) and once with diluted (10%) sodium bisulfite solution (40 mL). Finally, it was dried over anhydrous sodium sulfate, filtered and the solution concentrated at reduced pressure. As commercially available LDA-solutions contain ethylbenzene as co-solvent, the product was dried on high vacuum using a 90 °C oil bath. The product (9c) is a yellow oil (8.076 g, 95%), stored under nitrogen in the dark.

Synthesis of BICAAC precursor 12
Synthesis of BICAAC precursor salt 12 In a Schlenk-flask, imine 9c (7.086 g, 25.00 mmol) was dissolved in dry 1,4-dioxane (15 mL), followed by the addition of 1,4-dioxane solution of 3 molar hydrochloric acid (41.66 mL, 125.00 mmol) at room temperature. Alternatively, the HCl can be generated in-situ by reacting dry methanol with acetyl chloride in equimolar ratio in dry dioxane. The flask was then sealed and stirred at 80 °C for at least 48 hours. The formed solution or suspension was allowed to cool down to room temperature, then concentrated sodium bicarbonate solution (50 mL) and dichloromethane (80 mL) were added to neutralize the hydrochloric acid. The organic phase was separated, concentrated at reduced pressure, and then aqueous solution of ammonium tetrafluoroborate (50.00 mmol, 5.24 g, cca. 50 mL solution) was added to the dichloromethane solution. The resulted mixture was stirred for two hours, then separated, and the organic phase was evaporated affording the mixture of the unreacted starting material (9c) and the product. Adding diethyl ether to this mixture dissolved the impurities and induced the crystallization of the product. The crude BICAAC salt can be recrystallized from hot hexane, giving 12 as a pale-yellow powder.

Analytical data and NMR spectra of the respective iminium salts 10-13
Each iminium compound were synthetized following the procedure described above.

Synthesis of mono-BICAAC-Ru complexes 5, 14, 18 and 19
For complexations, the free carbenes were not isolated. Their in-situ preparation from the corresponding precursor salts and their subsequent complexation was a straightforward method. This method was already used for the synthesis of CAAC-5 systems. 7,8 General procedure for mono-BICAAC-Ru complexes Synthesis of mono-carbene complexes 5, 14, 18 and 19 In glovebox, Hoveyda-Grubbs first generation complex (HG1, 167 μmol, 100.0 mg) and the HBF4-salt of the BICAAC precursor (250 μmol, 96.5 mg 10, 88.8 mg 11, 92.7 mg 12, 99.7 mg 13) were measured into two different vials. Dry THF (5-5 mL) was added to both compounds forming a suspension (10-13) and a solution (HG1). The LiHMDS (1 M in THF, 275 μmol, 275 μL) was added to the stirred suspension of the BICAAC precursors at room temperature, resulting a clear solution immediately, indicating the formation of the free carbene. Depending on the substrate, it may be colorless (13), yellow (11) or brown (10 and 12). This solution was added to the stirred solution of the HG1 complex yielding a brownish solution. The conversion of the reaction can be followed by 1 H and 31 P NMR, and also visually, as the reaction is progressing the color of the mixture is gradually turning green. The reaction time is minimum 20 minutes (e.g. 13), but no more than 3 hours.
The resulted solution was filtered through an alumina pad and concentrated at reduced pressure. Depending on the crystallization ability of the complex, it can be purified by crystallization (14 and 5) or column chromatography (18 and 19). For crystallization, the crude material was dissolved in warm (40-50 °C) hexane (cca. 10 mL) to form a concentrated solution, then it was cooled to -20 °C (freezer). Green crystals were formed overnight. For the chromatography, the crude material was taken up in hexane under air, then it was layered on the top of a column. Hexane containing 0-50 vol.% ethyl acetate was used as eluent over alumina as stationary phase. The green fractions were collected and concentrated, affording the products as green solids upon evaporation of the solvent. The complexes are soluble in almost all organic solvents.

Synthesis of bis-BICAAC-Ru complexes 16 and 20
Following the synthetic procedure reported by our group recently, 7 bis-carbene complexes were prepared starting from the Grubbs 1 st generation complex (G1). The phosphine (tricyclohexylphosohine) -carbene exchange was carried out in glovebox using THF solution of the in-situ generated carbene by reacting the precursor salt with a strong base, lithium(hexamethyldisilyl)amide (LiHMDS).

Synthesis of bis-carbene complexes 16 and 20
In glovebox, Grubbs first generation complex (G1, 122 μmol, 100.0 mg) and the HBF4-salt of the BICAAC precursor (365 μmol, 141.0 mg 10, 130.3 mg 11) were measured into two different vials. 5-5 mL of dry THF was added to both compounds forming a suspension (10 or 11) and a solution of G1. The LiHMDS (1 M in THF, 402 μmol, 402 μL) was added to the stirred suspension of the BICAAC precursors at room temperature, resulting a clear solution immediately, indicating the formation of the free carbene. Depending on the substrate, it may be brown (10) or yellow (11). This solution was added to the stirred solution of the G1 complex yielding a brownish solution, which was stirred over 2 hours. The conversion of the reaction can be followed by 1 H and 31 P NMR. The resulted solution was filtered through an alumina pad, then it was concentrated at reduced pressure. The complex can be purified by column chromatography. The crude material was taken up in hexane and layered on top of the column under air. Hexane containing 0-50 vol.% ethyl acetate was used as eluent over alumina as stationary phase. The yellow fractions were collected and concentrated, affording the products as yellow solids upon evaporation of the solvent.

Synthesis of complex 15
An oven-dried Schlenk flask was charged with complex 14 (0.162 mmol, 100.0 mg) and dissolved in 2 mL of DCM. Then, it was cooled down to -30°C by using a mixture of acetone/dry ice. Methyl trifluormethanesulfonate (0.275 mmol, 30 μL) was added dropwise through a septum to the stirred DCM solution of 14. After 15 minutes, the flask was allowed to warm up to room temperature. An hour later, green precipitate formed. Evaporation of the solvent and the alkylation agent resulted the product (15) as green solid (106.3 mg, 84%).

Synthesis of ionic bis-BICAAC-Ru complex 17
Attempt for the synthesis of complex 17 An oven-dried Schlenk flask was charged with complex 16 (0.116 mmol, 100.0 mg) and dissolved in 2 mL of DCM. Then, it was cooled down to -30°C by using a mixture of acetone/dry ice. Methyl trifluormethanesulfonate (0.396 mmol, 43 μL) was added dropwise through a septum to the stirred DCM solution of 16. After 30 minutes, the flask was allowed to warm up to room temperature. No precipitate formed. Evaporation of the solvent and the alkylation agent resulted the crude product of 17 as yellow solid.
Although signals of the benzylidene positioned proton (Ru=CH) showed clear upshift (indicating full conversion of 16), an approximate 20mol% loss of carbene ligand was detected in 1 H NMR: a characteristic signal (singlet between 10 and 9 ppm) of protonated species -similar to precursor 10 -appeared. The formed side products may render similar chemically and physical properties to the target compound (e.g. the N-methylated derivative of 10), therefore the isolation and full characterization of the target complex did not succeed.

Representative example of the RCM of diethyl diallylmalonate (21) in toluene-d8 at 50 °C or 75 °C (0.05 mol% catalyst load)
In a glovebox, screwcap NMR tube was charged with 29 uL (0.12 mmol) diethyl diallylmalonate (21) and 0.6 mL toluene-d8. The sample temperature was equilibrated (at 50 or 75 °C) and the starting point was measured. The sample was removed from the spectrometer and a solution of the catalyst (14, 0.05 mol%, 0.038 mg in 30 uL toluene-d8) was added. The sample was placed back in the spectrometer and 1 H NMR transients were collected at every two minutes over 3 hours. Interestingly, the RCM kinetic curves can be described by eq. 1 (1) where dx/dt is the rate of formation of ring closing metathesis reaction (RCM) for product 22, x is the conversion value of 21 to 22. k is the rate constant, α and β are the reaction orders. Eq. 1. was solved numerically using MS Excel and the parameters of eq. 1 (i.e., k, α and β) were obtained by fitting of eq. 1. to the experimental x versus reaction time curves using the "Solver" function of MS Excel. The fitted curves along with the experimental ones are plotted in Fig. S38. According to the results of the fitting, the values of k, α and β for curve (a) were determined to be 0.035 min -1 , 0.51 and 2.09 respectively, while for curve (b) k = 0.039 min -1 , α = 0.55 and β = 2.14 were obtained. Thus, using values of α = 0.5 and β = 2, the kinetics can be approximated by eq. 2: (2) and integration of eq. 2 leads to eq. 3.

Representative example of the latent properties in the RCM of diethyl diallylmalonate (21) in toluene-d8 (0.25 mol% catalyst load)
In a glovebox, screwcap NMR tube was charged with 29 uL (0.12 mmol) diethyl diallylmalonate (21) and 0.6 mL toluene-d8. The sample temperature was equilibrated at 50 °C and the starting point was measured. The sample was removed from the spectrometer and a solution of the catalyst (14, 0.25 mol%, 0.188 mg in 37 uL toluene-d8) was added. The sample was placed back in the spectrometer and 1 H NMR transients were collected over 1 hours. In one hour, the temperature was increased to 75 °C and the measurement continued for a total of 6 hours.

RCM of diethyl diallylmalonate (21) with catalyst 5 in toluene-d8 at 100 °C
In a glovebox, a small vial was charged with 29 uL (0.12 mmol) diethyl diallylmalonate (21) and 0.6 mL toluene-d8. Catalyst 5 (5 mg) was dissolved in 4 mL of toluene-d8 and 29 uL of the catalyst solution (0.19 mg, 3*10 -4 mmol) was added to the reaction mixture. The vial was sealed and heated up to 100 °C for 12 h. After cooling to room temperature, the solution was measured via 1 H NMR, which showed no RCM product (22) formation.

Representative example of the ISOMET reaction of methyl oleate (23)
In a glovebox, Fischer-Porter bottle was charged with isomerization catalyst (RuH, 2 mol%, 15 mg), methyl oleate (240 mg, 0.80 mmol), toluene (3 mL) and the metathesis catalyst (14, 1 mol%, 5 mg). The bottle was closed in the glovebox under argon and transferred to the ethylene line, equipped with a pressure gauge. The line was vented 5 times with ethylene (99.9%) followed by the venting of the bottle 5 times. The ethylene pressure was set to 10 bar, the bottle was closed and heated to 75°C for 24 h. After the reaction mixture cooled to room temperature, sample of the gas phase was collected and measured via GC-FID. Part of the liquid phase was measured via GC-MS in the presence of anthracene as internal standard.

Representative example for the CM of 1-decene with catalyst 14 in neat at 75 °C
In a glovebox, a small vial was charged with 5.4 mL (28.5 mmol) 1-decene. Solution of catalyst 14 (0.18 mg, 10 ppm) in toluene was added to 1-decene and the mixture was stirred for 3 h at 75 °C. After cooling to room temperature, part of the mixture was dissolved in toluene-d8 and measured via 1 H NMR.

X-ray structure determination
The crystal and molecular structures of compounds 14, 18, 20 and 21 were established unequivocally by single-crystal X-ray diffraction analysis (Figure 1). The results for the X-ray diffraction structure determinations were very good according to the Checkcif functionality of PLATON software (Utrecht University, Utrecht, The Netherlands). 4 The reasons for alert A and B level errors are the presence of very heavy element (ruthenium) and the highly irregular shape (very thin plates or needles) of the crystals. However, the deposited Crystallographic Information File (CIF) includes answers for these errors and the overall correctness of the structures is not influenced.
In case of 20 because of the unusually long unit cell axis of 59.4 Å Cu Kα radiation was applied. In 21 there is a half distorted solvent hexane molecule in the asymmetric unit. Further experimental details of the crystal parameters, data collection, and results of structure refinement are given in Table S1. The very rigid BICAAC ligand induce strong constrains on the coordination of ruthenium. Selected bond length and angle data of ruthenium coordination are compiled in Table S2.
The coordination of the ruthenium center in BICAAC-Ru complexes is similar to NHC and CAAC-Ru complexes having the ruthenium and two chlorides is approximately in one plane with the coordinating benzylidene carbon, while the NHC, CAAC or BICAAC carbene and the coordinated oxygen are above and under the plane, respectively (Table S2). The C-Ru-C angles of the two BICAAC-Ru complexes are close to 105 o which values are in alignment with those of the reported six-membered CAAC-6 Ru complexes. 9 However, the relative configuration of the stereogenic centers of the BICAAC ligand is the most interesting feature of the structure of these new complexes. In our case, the bicyclic nature of the BICAAC ligand decreases the number of possible diastereomers and it is also important how the crystal mirrors the achiral route of the synthesis. In 14 we found two molecules in the asymmetric unit which are mirror images of each-others with slight difference in the conformation of the phenyl rings ( Figure S43). Here the glide plane of the lattice symmetry generates the opposite enantiomer of both conformer as it is expected for a racemate and the crystal is also a twin by inversion. In 5 there are three complexes in the asymmetric unit and the ratio of the enantiomers is 2:1, their overlayed structure is shown at Figure S44. Moreover, as the space group is centrosymmetric the final ratio of the enantiomers is 1:1, as it is expected for the racemate. The structure of complex 18 is the most interesting in this respect. In this case the space group (No.19) is noncentrosymmetric, Sohncke space group suitable for pure enantiomers. We found racemic conglomerate i.e. mechanical mixture of enantiomer pure crystals as well as racemic twin and also crystalline powder. For the chiral crystals the Flack parameter was very close to 0 with slight difference in conformation ( Figure S45) while the inversion twin crystal gave Flack parameter of 0.5, as it is expected.     (8) 1.962 (9) 1.929 (9) 1.936 (9) 1.921 (9) 2.140 (4) 2.123 (4) Cbz -Ru 1.818 (8) 1.817 (8) 1.822 (9) 1.832 (9) 1.828 (9) 1.842 (9) 1.829 (4